This is probably the best gardens in central Tokyo and it is well worth spending an hour or two here. It is located next to the Tsukiji Fish Market and within walking distance from Shinbashi Station (Ginza subway line and JR Yamanote line). Entrance is yen 300 and you will receive an English map with some description of the sights.
The garden was the official hunting ground for the Tokugawa Shogunate in the 17th century and the upper classes hunted ducks here until the Second World War. There are some description and photographs of how this was done with nets rather than other kinds of lethal weapons. There is also a salt-water pond with three islands linked by wooden bridges. Green tea is served in a rest house on one of the islands. The pond is swarmed with ducks and other birds if you visit late afternoon.
The park offers fantastic views of Rainbow Bridge as well as interesting vistas of very modern high-rise buildings in the background. Many new buildings in the vicinity are currently under construction.
Vending machines are available at the kiosks and picnicking is allowed in several areas. All paths are gravel, which is not convenient for strollers.
The park can also be reach by waterbus from the Sumida River Line from Asakusa.
A bonus if you drive in Tokyo is a rare free parking area - although the 40 odd spaces tend to fill up fast.